I'm currently at the NNg Usability Week here in Amsterdam, and have just taken part in a usability test on their exam certification program. It's still early stages, so it may change - but I thought it was interesting enough to share:
If you have taken part in any training, then you can take a test - (approx $50 per test), if you complete 5 tests then you get a certification. If you complete 15 tests then you get a 'master certification', you can also choose 4 paths if you wish to specialise in certain areas, eg UX Research, Mobile, (can't remember the other two paths).
If you have taken part in a previous NNg training course, then they are backdating this offer for a certain period (not sure of period but a few years was mentioned).
Might be an interesting one to look out for!